1. Technical Field
The present invention relates to an imaging lens, a camera and a personal digital assistant.
2. Description of the Related Art
There are a variety of functions and configurations used in a widespread digital camera. In recent years, a relatively large imaging element having about 20-45 mm diagonal length is used in such a digital camera, and a high image quality and compact camera equipped with a high-performance single focus lens has greatly drawn user's attention.
Most of user's requests relate to portability, namely, compactness as well as a high performance. A high performance requires at least a small collapse of a point image till a peripheral part of a field angle at high contrast with small coma flare in an opened state of an aperture stop, non-generation of unnecessary coloring in a portion having a large brightness difference with small chromatic aberration, and small distortion, in addition to a resolution corresponding to an imaging element having 10-20 million pixels.
Moreover, compactness requires the entire length when normalizing with a focal length or the maximum image height shorter than that when using a small imaging element because an actual focal length is increased due to a relatively large imaging element.
A large aperture at least less than F 2.8 is also required in view of differentiation from a general compact camera equipped with a zoom lens.
Many users desire a certain level of wide-field angle, and it is preferable for a half-field angle of an imaging lens to be 28° or more. A half-field angle of 28° corresponds to about 41 mm focal length converted into a 35 mm film camera (so-called Leica size).
An image from an imaging lens is imaged by an imaging element (area sensor) in a digital camera. In view of the property of the area sensor having a color filter and a micro lens with respect to each pixel, various retrofocus type lenses in which a lens group having a negative refractive power is arranged on the object side and a lens group having a positive refractive power is arranged on the image side are proposed as a wide-angle single focus lens which moves the exit pupil position away from the image face and is suitable for entering a peripheral light beam onto the sensor at an angle close to the perpendicular.
However, such a retrofocus type increases an entire length (distance from most object-side face to image face) of a lens, so that a digital camera can not be easily downsized.
In recent years, owing to an improvement in and optimization of an on-chip micro lens and progress in an image process in an imaging element having a relatively large diagonal length of about 20-45 mm, a certain level of oblique incidence of a peripheral light beam onto an imaging face is allowed.
That is, a system which can sufficinently allow up to about 30° in an angle between a main light beam and an optical axis in the maximum image height can be created.
If such a system is used, a lens type suitable for downsizing can be selected without regard to the vertical incidence of the peripheral light beam.
A lens type which is more suitable for downsizing than the retrofocus type includes an approximate symmetric type and a telephoto type in which a lens group having a negative refractive power is arranged on an image side.
An imaging lens of this type is described in Patent Documents 1 (Japanese Patent Application Publication No. S63-024213), 2 (Japanese Patent Application Publication No. H09-236746), 3 (Japanese Patent Application Publication No. 2000-321490), 4 (Japanese Patent Application Publication No. 2005-352060), 5 (Japanese Patent Application Publication No. 2009-216858), for example.
An imaging lens described in Patent Document 1 is a telephoto type having a four-group and four-lens configuration which is widely used in a compact film camera. It is small but it includes large distortion and astigmatism.
An imaging lens described in Patent Documents 2, 4 is small and has a high imaging performance, but it has 35° or more between the main light beam and the optical axis in the maximum image height. Because of this, it has a problem for use in a digital camera even if it is combined with the above-described imaging element.
An imaging lens described in Patent Document 3 has a large aperture but it includes many lenses, creating rising costs. An imaging lens described in Patent Document 5 has a large entire length compared with a focal length, so that it has a problem in downsizing.
An imaging lens described in Patent Documents 3-5 does not have a relatively large air space inside a lens system, and a large back focal length. For this reason, it is difficult to reduce the thickness of the imaging lens for housing even if a collapsed mechanism is used; thus, it is also difficult to downsize the imaging lens.